1. Field of the Invention
The invention relates to a doctor roller apparatus for the metered coating of a printing machine roller with a liquid application medium, in particular for coating an ink applicator roller with printing ink, of the type having at least one doctor roller supported by a doctor roller bed and placed with its working surface against the printing machine roller.
2. Description of the Related Art
Such a doctor roller apparatus is known from U.S. Pat. No. 5,372,644, in which the ink applicator roller is connected via a nip to a transfer roller, the surface of which dips into a fountain filled with application medium. The fountain is sealed off relative to the transfer roller by means of a barrier doctor blade and a doctor roller. The doctor roller does not cause the fountain to be sealed off completely, since it has an engraving through which a metered quantity of the application medium can flow, this quantity remaining on the surface of the transfer roller. This quantity of application medium defined by the respective engraving is then transferred from the transfer roller onto the applicator roller.
To vary the metering, a different doctor roller with an appropriately adapted engraving must be used, this being a disadvantage in view of the associated outlay in terms of production and stockkeeping.
By contrast, the object of the present invention is to provide an apparatus of the type mentioned in the introduction, by means of which a self-regulating supply of the printing machine roller with application means and rapid adaptation to changed metering conditions are possible. Furthermore, the doctor roller apparatus according to the invention is to be capable of being produced cost-effectively.
This object is achieved by an apparatus having a pressure source which supplies the application medium to a clearance between the doctor roller and the doctor roller bed, and a pressure regulating valve between the source and the doctor roller.
The metering of application medium on the printing machine roller, in particular of printing ink on an ink applicator roller, takes place by means of the doctor roller apparatus according to the invention in a self-regulating manner. For example, when the printing ink requirement on the ink applicator roller is very low, because the latter has a high proportion of ink-repelling surfaces, only a small part of the application medium transported by the doctor roller remains adhering to the ink applicator roller, while the rest is conveyed back into the clearance in which a higher pressure is then established. This pressure prevails on the outlet side at the pressure-regulating valve which thereupon restricts the further inflow of printing ink in order to lower the pressure in the clearance. As a result of the sliding mounting of the doctor roller due to the printing ink forming a hydrodynamic lubricating film in the clearance, a kind of consumption lubrication occurs, in which excess printing ink is automatically conveyed back into the clearance by the doctor roller. This results in a closed system with a low-loss ink circulation and the use of only a small amount of solvent. The application, metering and smoothing of the ink on the ink applicator roller are carried out advantageously by means of a single unit.
On account of the double function of the clearance which gives rise, on the one hand, to a sliding mounting for the doctor roller and, on the other hand, to a transport duct for the printing ink, the doctor roller apparatus according to the invention has an extremely compact build. A short inking zone is consequently obtained, with the result that the outlay in cleaning terms is reduced. Since there is in the clearance a sliding film consisting of printing ink, the mounting of the doctor roller has some elasticity, so that axial errors, unavoidable in practice, between the doctor roller and the ink applicator roller can be compensated. Thus, advantageously, a surface pressure which is constant over the axial length can be achieved between the doctor roller and the ink applicator roller. Finally, the doctor roller apparatus according to the invention can be used for different circumferences and types of printing machine rollers.
According to a preferred embodiment, the working surface of the doctor roller is provided on its ends with helical conveying grooves oriented so that, when the doctor roller is driven, printing ink located at the lateral ends of the clearance can be conveyed to the middle of the doctor roller. Application medium is thereby prevented from escaping laterally from the clearance.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.